A forged piston is made by stamping the material in a forging press. A cast piston is made form liquid alloy forced at high pressure into a precision steel mould. Forging alloys and casting alloys are not the same.

The forging process physically squeezes and deforms the material so as to produce grain flow patterns in the material that suit the stresses on the material in service whereas the grain structure in casting has no distinct pattern in any direction. Consequently the forging process gives a far stronger piston. This is important in race engines because the pin boss, ring lands and crown are potentially weak zones. Forging alloys have a much higher fatigue resistance as well, in other words they will withstand far more load 'cycles' at a given stress level.

If the accelerative loads around tdc are high enough the cast piston will break up, usually at the pin boss region, but sometimes, depending on the local stress, between the upper part of the pin boss and the crown. This can happen almost instantly from gross overload at too-high rpm or from fatigue - over a period of some hours use. You have to know the limits. I do.

Many cast pistons have cast-in steel struts for quiet running, again, this is an intrusion into the homogeneity of the material and a major weak point. To put it simply - forging gives strength in the right places. This reduces the 'k factor' - a compensation for stress concentrations that must be included in any engine stress evluation and which makes the piston weaker than simple load calculations would indicate

The Excel data (from actual parts) below shows that the RR58 forging alloy material is not only inherently stronger at working temperature but it can withstand far higher engine speeds without overload. Even at a low 6800 the cast item is well on the way to failure. And they would break up in this engine if I used them. My forged piston design is nowhere near failure even 7800 rev/min.

A forged piston can be lighter in some areas than a cast one needs to be to achieve the required strength and it is possible, because they are made in small batches, to design the strength 'where it is needed' and leave other less critical regions light. For example it is possible to arch the under-crown region to distribute the bending stresses evenly, and make the pin boss region stronger. A forged piston is only as good as the design and manufacture. I have seen a lot of forged pistons that are massively made, no attempt at lightweight design - way too heavy everwhere.

I do stress computations on many of my pistons, check the stress and fatigue data, match the characteristics against know failure modes and try and design them with minimum weight in mind.

Attachments

Two of many of my forged piston designs, by CP in California, left - medium high comp dome and right, high comp, would not use bigger on the 8v, and CR does depend on bore size.